1. Field of the Invention
The present invention relates to a method of forming patterns, and more specifically to a method of forming fine patterns, useful in producing e.g., various semiconductor devices and magnetic bubble memory elements, by utilizing both the electron beam lithographic exposure method and the ultraviolet light exposure method.
2. Description of the Prior Art
The trend toward high densities and high integration degrees in modern magnetic bubble memory elements and semiconductor integrated circuits has been placing increasing demands for the development of methods which are capable of accurately and easily forming fine patterns having very fine line widths and gaps.
So far, a variety of patterns of magnetic bubble memory elements and semiconductor integrated circuits have, in general, been formed by the method called photolithography.
With the conventional photolithography employing ultraviolet light of a wavelength of about 320 to 500 nm, however, it is difficult to form fine patterns maintaining high precision due to the effects of diffraction and interference that are characteristics inherent in the light. In order to accurately form fine patterns without adverse effects due to the wavelengths of the light, there has been proposed a so-called electron beam lithographic method which employs electron beams having wavelengths which are very much shorter than the wavelengths of ultraviolet light, instead of employing the ultraviolet light.
The wavelengths of the electron beam are so much shorter than the wavelengths of the ultraviolet light that adverse effects caused by the interference and diffraction can be neglected, and very fine patterns can be formed while maintaining precision.
According to electron beam lithography, a pattern is described by the electron beam on a resist film applied onto a workpiece (such as metal film, silicon oxide film or the like), and is subjected to development to form a desired resist pattern.
By using the resist pattern as a mask, the underlying workpiece is processed by etching to form a pattern of the workpiece. With a so-called wet etching employing an etching solution, however, it is difficult to accurately form fine patterns of the order of smaller than 1 .mu.m. Therefore, a dry etching method is employed by utilizing sputtering properties of low-temperature plasma or high-speed ions. Therefore, the resist film exposed to the electron beam must have high resolving power, and must not be deteriorated or deformed by the dry etching.
At present, however, there is no resist film for exposure to an electron beam that has high resolving power and excellent resistance against the dry etching; this fact hinders the development of electron beam lithographic methods.
For example, the PMMA (polymethyl methacrylate), which is a positive-type resist for exposure to an electron beam, has good resolving power and is capable of forming a resist pattern having a width of about 0.8 .mu.m. However, if the PSG film (phosphorus glass film) is subjected to the etching by the reactive sputtering using C.sub.4 F.sub.8 and helium gases with the PMMA as a mask, abnormal wrinkles develop on the whole surfaces of the resist film. Namely, the pattern is deformed by the wrinkles, and it becomes difficult to obtain a good resist pattern.
The PGMA (polyglycidyl methacrylate), which is a negative resist for exposure to an electron beam gives a resolving power of up to about 1.2 .mu.m. When the aluminum film is subjected to etching by reactive sputtering using BCl.sub.3 gas with the PGMA as a mask, however, the thickness of the PGMA must be increased since it has a large etching ratio. Consequently, it is impossible to realize the resolving power of the order of 1 .mu.m.
Moreover, although various resists for exposure to an electron beam have been developed, there is still available no resist film which has high resolving power and resistance against dry etching.
Therefore, it is difficult to form good fine patterns by electron beam lithography unless some contrivance is made to the process of electron beam lithography.
With electron lithography, furthermore, the portions to be exposed must be successively scanned by the electron beam which is finely converged. Therefore, the scanning requires extended periods of time, and the produceability is small.